"If someone has allergies to more than one tree, say ash and birch, which is not that uncommon to have allergies to those two trees, they're showing up together," said Coates. "So people are getting a double-whammy from their allergies."

Hot weather generally creates a shorter tree pollen season with higher counts, meaning people with those allergies don't get sick for as long, she said.

"I hear that every year that everybody says it's the worst season," said Coates.

Dr. Lee gives Kirsten Cornelson her first allergy shot. (CBC)

After 20 years of collecting data, Coates see no clear overall patterns, because every tree species reacts differently to weather conditions each year.